About 1 in 12 men sees color differently — most often, reds and greens that collapse into the same muddy tone. It’s not seeing in black and white; it’s certain colors becoming impossible to tell apart. Switch the scene below between types of color vision deficiency, and watch how much “red means stop, green means go” quietly assumes.
Color vision deficiency (CVD) is almost always genetic and present from birth. The most common forms are red-green — a weakness in the cone cells that detect red or green light — which makes those hues blur together. True “no color at all” (achromatopsia) is rare. Because the common genes sit on the X chromosome, it affects about 1 in 12 men and 1 in 200 women.
People with CVD aren’t “getting colors wrong” — their eyes genuinely receive less color information, and they adapt brilliantly using brightness, position, and context. The problem isn’t them; it’s designs that assume everyone sees the same red and green.
The design lesson is simple: never rely on color alone. A red “error” dot and a green “success” dot can look identical. “Tap the green button” or a red-and-green chart can be unreadable. Pair color with text, shape, icons, or labels — and check your work in a simulator like this one.
“This is roughly what I see — red and green look the same to me. It really helps when things aren’t labeled by color alone.”
Simulation uses standard color-transform matrices (the same family used across accessibility tooling). They approximate, not reproduce, individual color vision.